Use a comma to separate two independent clauses joined by a conjunction. Independent clauses are those that can stand alone as sentences if they had to -- meaning they contain at least a subject and a verb. The Purdue University Online Writing Lab offers this example: "I am going home, and I intend to stay there."
Insert commas between lists of three or more. For example: "She will split the money between her sister, brother and daughter." Some grammarians choose to add a second comma before the word "and" in sentences such as this one, but using that comma has become a matter of preference rather than an hard-and-fast rule.
Use commas when an "ly" adjective is used with other adjectives. "The Blue Book of Punctuation and Grammar" gives this example: "Felix was a lonely, confused boy." Be careful here not to confuse "ly" adverbs with adjectives as "ly" adverbs don't necessarily need commas after them.
Use a comma inside quotation marks at the conclusion of a quote if you are adding attribution at the end. For example: "She likes to run and play," he said.
Use a semicolon between two closely related independent clauses. Grammar Girl gives this example: "I have a big test tomorrow; I can't go out tonight."
Place semicolons in sentences with complex lists. Grammar Girl gives this example: "This week's winners are Joe from Reno, Nevada; Diane from Phoenix, Arizona; and Matt from Irvine, California."
Use a semicolon to create variety in your sentences, especially if you have a lot of choppy sentences in a row. Place a coordinating conjunction between the two sentences you bring together with the semicolon. Here's another example from Grammar Girl: "If you want me to go out tonight, you need to help me with my homework first; and if you say no, I'll know that you don't really care about going out."